In order to increase channel rates despite limitations of bandwidth of analog to digital converters and digital to analog converters, as well as digital signal processors (DSP), multiplexing several subcarriers inside one channel is being used. For instance, 400 Gb/s signals are realized recently with two subcarriers at 200 Gb/s rate each. Here, these two subcarriers are optical signals which are multiplexed in frequency and considered as part of one channel.
Unmodulated spectral spacing between several channels or subcarriers is not used to transmit information. Accordingly, squeezing channels and reducing the space between subcarriers or carriers is an efficient way to increase the data transmitted in optical systems, and therefore to reduce CAPEX (capital expenditure) investment. For instance, using Nyquist spectrally shaped subcarriers can improve spectral efficiency and therefore channel capacity. In principle, it enables to reduce the bandwidth occupied by a subcarrier down to the symbol rate of the channel, and therefore to increase the spectral efficiency and in fine the capacity of the optical system if more subcarriers are multiplexed in the same bandwidth. Nyquist Spectrally shaped subcarriers can be called Nyquist channels, Nyquist WDM (wavelength division multiplexing), Nyquist shaped, or Nyquist pulse channels, depending on sources.
Moreover, a shaping technique for subcarriers is known as Sub-Nyquist, which can be called Faster-Than-Nyquist, Super-Nyquist, or optical time frequency packing, depending on sources. It enables to reduce the bandwidth occupied by the subcarrier or channel below its symbol rate. In such techniques, the spacing between subcarriers or channels becomes very tight, and a very little space is left unused between subcarriers, typically a few gigahertzes for current optical systems.
Optical telecom systems are used over long period of time, typically more than ten years in changing environment. Such constraint has an influence on the characteristics of the components of the system including light sources. For instance, tunable lasers can be used to generate subcarriers or channels in optical networks with frequency stability from ±1.25 GHz to ±2.5 GHz, specified at end of life, depending on conditions. The frequencies, over which two such lasers are susceptible to relative variation in that cases, are respectively 2.5 GHz and 5 GHz, each of which is larger than the spacing between channels or subcarriers in ultra-dense systems. In such an eventuality, neighboring channels of subcarriers would overlap, which causes linear crosstalk among them, lowering the quality of received signal, and finally reducing system performance or transmission distance. The very high frequency of optical carriers, however, excludes the possibility to monitor directly the deviation of frequency of channels and relative deviations between channels.
An example of a multicarrier optical transmitter is described in patent literature 1 (PTL1) which can control a frequency interval between subcarriers. The multicarrier optical transmitter described in PTL1 includes transmission CW (continuous wave) light sources, monitor light extraction means, light vector modulation means, optical multiplexing means, frequency interval monitor means, and driving signal generating means.
The transmission CW light sources output the CW light having optical frequency corresponding to each subchannel. The monitor light extraction means tap part of light output from the transmission CW light sources and send it to the frequency interval monitor means as monitor light. The frequency interval monitor means detects the frequency interval between the output light of the transmission CW light sources from the monitor light, generates frequency interval information including beat signals from the detected frequency interval, and sends it to the driving signal generating means. The driving signal generating means generates driving signals for the light vector modulation means based on the frequency interval information and transmission data signals inputted from the outside. The light vector modulation means generate subchannel signals based on the inputted driving signals and the output light of the transmission CW light sources so that the subchannel signals may have a predetermined frequency interval. The optical multiplexing means multiplexes the subchannel signals output from the light vector modulation means. The multiplexed subchannel signals are output as multicarrier optical signals from an output port.
It is said that the multicarrier optical transmitter described in PTL1 can perform feedforward compensation for a deviation of the frequency interval between subcarriers from an intended value.